The present invention relates to an adaptive noise reduction apparatus, and more particularly to an apparatus in a MUSE decoder for a high definition television (HDTV) system, which uses a motion detection signal. A noise reduction function for an image signal can be always be performed at its maximum in still pictures, so as to obtain the image signal having a amount of analog noise. However, in motion pictures, the noise reduction function can be performed only in a predetermined noise level above, so as to prevent an afterimage.
According to the conventional noise reduction apparatus for a MUSE decoder shown in FIG. 1, a subtractor 20 receives an 8-bit noise reduced signal a, being a final output signal of the above apparatus, as a feedback signal via a horizontal motion compensator 10. Subtrator 20 also receives an 8-bit current frame image signal b from a de-emphasis unit (not shown) and subtracts the latter from the former, and transmits the result to a current frame value altering unit 40. A memory 30 produces a signal representing the desired amount of noise reduction upon reception of a non-linear edge signal d from a memory switch (not shown) and a noise level detection signal e from a noise level detector (not shown), and transmits the produced signal to current frame value altering unit 40. Thus, current frame value altering unit 40 alters the value of the subtracted image signal from subtractor 20 to an appropriate value in accordance with the noise reduction signal from memory 30.
An adder 50 receives and adds the altered current frame value from current frame value altering unit 40 and the 8-bit current frame image signal b from the de-emphasis unit (not shown) to output a noise reduced signal a. This operation allows an adequate low-pass filtering effect to be obtained. In FIG. 1, a signal c is a select signal for carrying out the noise reduction function either automatically or manually.
As described above with respect to the conventional noise reduction apparatus, since noise reduction function cannot be performed below a certain noise level, the analog noise of the image's background cannot be eliminated in a still picture. Further, afterimage occurs in a motion picture when the noise reduction function is forcibly facilitated by manual manipulation resulting in a bad picture.